Voice inversion scrambling is an analog method of obscuring the content of a transmission.
It is often easy to guess what is happening in the conversation by listening for other audio cues like questions, short responses and other language cadences.
This can be done by amplitude modulating the speech signal with the carrier, then applying a low-pass filter to select the lower sideband.
[1] This will make the low tones of the voice sound like high ones and vice versa.
There are more advanced forms of voice inversion which are more complex and require more effort to descramble.
A rolling code can also be added to this method for variable split band inversion (VSB).